This invention relates to a magnetron tube, more particularly an improved connecting structure between the cathode stem and a filter device of a magnetron tube provided with a filter device for preventing leakage of unwanted radiation which causes noises in television receivers.
As disclosed in U.S. Pat. No. 3,987,333, for example, the stem structure of the magnetron tube of the type just described comprises a pair of input terminals connected to the cathode electrode through supports arranged coaxially with the cathode electrode, and spaced from each other by an insulator. The pair of input terminals is connected to the lead terminals of the filter device.
To insulate various elements of the magnetron tube, ceramic insulators are generally used in view of their heat resistant property and excellent high frequency characteristic. For this reason, ceramic insulators are also used for insulating input terminals from each other or insulating them from other members.
On the other hand, each input terminal is made of Kovar (trade name) having substantially the same thermal expansion coefficient as the ceramic insulator so as to prevent various troubles caused by the difference in the thermal expansion coefficients. However, lead wires which are connected to the input terminals are generally made of copper. The melting point of copper is 1083.degree. C. which is lower than the melting point 1450.degree. C. of Kovar so that when the lead wires of the filter device are connected to the input terminals, for example by arc welding, the tips of the lead wires to be welded melt prior to the input terminals, and the molten copper becomes spherical which surrounds the ends of the input terminals prior to the melting of Kovar thus preventing satisfactory welding between the copper lead wires and Kovar input terminals.
It is also difficult to correctly position the lead wires of the filter device with respect to the input terminals, during welding operation. More particularly, as the surface area of the input terminal is considerably larger than the diameter of the lead wires it is difficult to obtain satisfactory welds unless contacting the lead wire to the input terminals over large areas as far as possible and firmly holding the contacted assembly. However, such welding operation is usually performed while urging the lead wires against the input terminals by hands so that it is difficult to maintain this condition until both the lead wires and the input terminals completely melt.
Where the lead wires are secured to the input terminals of the stem structure by using a special jig the mounting position of the lead wires on the stem structure would be fixed. When welding is carried out under these circumstances the result of welding becomes poor. Even when the lead wires are satisfactorily welded to the input terminals, the crystal grains at portions of the lead wires near the welds become large due to the heat of welding thus causing the welds to become brittle and fragile. Accordingly, when the lead wires are merely welded, the portions of the lead wires near the welds would be broken by vibration and shock.